1. Technical Field of the Invention
The present invention relates to a spark plug for internal combustion engines which are mounted in structures such as vehicles.
2. Related Art
Spark plugs are used as igniting means in internal combustion engines, such as the engines for vehicles. Some of such spark plugs have a configuration in which a center electrode is permitted to axially face a ground electrode to form a spark discharge gap therebetween. This type of spark plug causes discharge in the spark discharge gap to use the discharge for the ignition of the air-fuel mixture in the combustion chamber.
In the combustion chamber, a flow of the air-fuel mixture, such as a swirl flow or a tumble flow, is formed. The flow is appropriately directed to the spark discharge gap as well to ensure ignitability.
However, depending on the mounting posture of a spark plug with respect to the internal combustion engine, a part of the ground electrode connected to a top end portion of a housing may be located upstream of the spark discharge gap. In this case, the flow in the internal combustion engine may be blocked by the ground electrode to stagnate the flow in the vicinity of the spark discharge gap. As a result, the ignitability of the spark plug may be impaired. In other words, depending on the mounting posture of the spark plug with respect to the internal combustion engine, the ignitability of the spark plug may be problematically varied. In recent years, in particular, there is a trend of using lean-burn internal combustion engines. In such an internal combustion engine, combustion stability may be impaired depending on the mounting posture of the spark plug.
Further, it is difficult to control the mounting posture of the spark plug with respect to the internal combustion engine, i.e. to control the position of the ground electrode in the circumferential direction of the spark plug. This is because the mounting posture of the spark plug is unavoidably varied, depending such as on the state of the mounting screws formed in the housing, or the degree of tightening the spark plug in the work of mounting the spark plug on the internal combustion engine.
In order to suppress the airflow from being blocked by the ground electrode, a patent document JP-A-H09-148045 discloses a configuration in which the ground electrode is drilled to form a hole therein, or a configuration in which the ground electrode is joined to the housing using a plurality of thin-plate members.
However, the “configuration in which the ground electrode is drilled to form a hole therein” as disclosed in the patent document JP-A-H09-148045 may weaken the strength of the ground electrode. If the ground electrode is thickened to recover the weakened strength, the thickened ground electrode after all may tend to block the flow of the air-fuel mixture.
Further, the “configuration in which the ground electrode is joined to the housing using a plurality of thin-plate members” as disclosed in the patent document JP-A-H09-148045 may complicate the shape of the ground electrode and the number of manufacturing steps may be increased, leading to the problem of increasing the manufacturing cost.